a) Field of the Invention
The present invention relates to a wet treatment or process, and more particularly to a wet process suitable for mirror etching a group II-VI compound semiconductor containing Zn as a group II element and a mixed crystal containing such a compound semiconductor which are expected to be used as the material of light emitting elements, and to a method of manufacturing a semiconductor device with a mirror etching process.
In this specification and claims, sulfuric acid or H.sub.2 SO.sub.4 means conc sulfuric acid (96 mol % H.sub.2 SO.sub.4) and hydrogen peroxide or H.sub.2 O.sub.2 means 30 mol % H.sub.2 O.sub.2. The reminder composition is H.sub.2 O.
b) Description of the Related Art
Etchants for a group II-VI compound semiconductor ZnSe are known, such as sodium hydroxide (NaOH) aqueous solution, bromine (Br) methanol solution, and ammonium hydroxide (NH.sub.4 OH) hydrogen peroxide (H.sub.2 O) solution.
In the case of NaOH aqueous solution, NaOH aqueous solution having 5 to 50 weight % is heated up to 80.degree. to 100.degree. C. for etching ZnSe. However, NaOH aqueous solution is anisotropic etchant so that etch pits are generated and mirror exchange is difficult.
In the case of Br methanol solution, Br is dissolved in methanol to 0.5 to 5 weight % for etching ZnSe. However, Br ethanol solution is an etchant which generates pits, and hence mirror etching is difficult. Furthermore, an etching speed (or rate) of a high quality crystal is slow. Still further, this etchant contains a volatile component so that the composition of the etchant may be degraded or made inconsistent.
In the case of NH.sub.4 OH-H.sub.2 O.sub.2 solution, ZnSe is etched by solution having a volume ratio of NH.sub.4 OH : H.sub.2 O.sub.2 =2 : 1 to 6 : 1. Although NH.sub.4 OH: H.sub.2 O.sub.2 solution is effective for the (1 1 1) B plane of ZnSe, the (1 1 1) A plane of ZnSe is hardly etched and ineffective.
Furthermore, since a robust oxide film is formed during etching by NH.sub.4 OH : H.sub.2 O.sub.2 solution, the etching speed lowers as the etching continues for a long time, being unable to etch ZnSe at a uniform etching speed.
As described above, of conventional ZnSe etchants, NaOH solution and Br methanol solution are difficult for use in mirror etching because these solutions are anisotropic depending upon a crystallographic plane orientation of ZnSe.
NH.sub.4 OH : H.sub.2 O.sub.2 solution is also not suitable for mirror etching because this solution can hardly etch the (1 1 1 ) A plane of ZnSe and the etching speed cannot be maintained constant.
Wafers of group II-VI compound semiconductor such as ZnSe, ZnTe, and ZnS presently available are still small in size. A Si wafer having a small size is also used for experiments or the like. Semiconductor wafers having such a relatively small size have been wet processed in simple manners described hereinunder.
FIG. 6A illustrates a simplest method. A wafer 53 of a II-VI compound semiconductor is placed on the bottom of a beaker 51, by directing its surface to be wet processed upward. An etchant 52 such as solution of aqua regia, sodium hydroxide, and Br methanol is poured into the beaker 51. A worker holds the beaker 51 with a hand and shakes it to rotate or agitate the etchant to etch the wafer 53. After the etching, the etchant is discarded and the wafer is rinsed by a great amount of pure water.
FIG. 6B illustrates another method using a wafer mount jig. A wafer 53 is placed in a basket 54 having a sufficient amount of liquid draining holes. After an etchant 52 is poured into a beaker 51, the lower portion of the basket jig 54 is immersed into the etchant 52. The basket jig 54 is moved up and down with a hand to move the etchant uniformly on the surface of the wafer 53. After the etching, the basket 54 is picked out of the beaker 51 and placed in another beaker in which pure water is overflowing.
The above-described simple wet etchings are difficult to form a stable flow of etchant. Since ZnSe of group II-VI compound semiconductor crystal has a strong ionicity, the etching state changes with even a small change of etchant component and mirror etching is disturbed.
Since the speed of a wet treatment is governed by an etchant flow regulated by a manual work, it is difficult to stably perform a wet process with a good reproductivity. In an etching process, variations of an etching speed and an etching depth are generated. The bottom surface of a wafer placed on the bottom of a beaker directly contacts the latter. In this case, a sufficient supply of etchant is difficult, and a wet treatment such as surface cleaning or the like by etching is performed substantially only for the upper surface of a wafer.
It is difficult to place a plurality of wafers on the bottom of a single beaker. If a wafer lies on the top of another wafer or collides with another wafer while etchant is agitated, variations of etching are generated and reproductivity lowers.
If a wafer is placed on the bottom of a beaker and etchant is moved round in the beaker, the wafer rubs the bottom of the beaker and collides against the side wall of the beaker. Such mechanical shocks may form defects in the wafer crystal.